Forecasters predict sub-zero temperatures will persist over the coming days, hampering efforts to free up Britain's frozen transport network.
The travel plans for many thousands of people are already in ruins, with airport operator BAA warning that Heathrow faces more delays and cancellations until potentially beyond Christmas Day.
There is no immediate respite in sight with temperatures in Northern Ireland again expected to drop to lows of -15 Celsius overnight and forecasters predicting the icy conditions will last for the rest of the week.
Growing frustration at how the statutory agencies have dealt with the wintry weather has led to calls for a special task force to be created in Northern Ireland to deal with gritting.
At Gatwick Airport, planes were grounded until 6am today as Britain continued to suffer from the big freeze.
There were also warnings of more long queues for Eurostar passengers at St Pancras station in London, after thousands were forced to wait for up to eight hours yesterday in bitterly cold weather.
Last night first-aiders from St John Ambulance were sent in as a precaution while people shivered in lines that stretched around the terminal building.
Drivers face widespread icy roads today, with temperatures expected to fall to -17C in the north of England overnight, and parts of the south dropping to -8C.
Transport Secretary Philip Hammond relaxed the rules on night flights yesterday in a bid to ease the passenger backlog at Heathrow.
The Government has agreed to relax night flights, allowing for arrivals until 1am until Christmas. And flights into London will be allowed to operate 24 hours a day.
But despite the moves, the airport was likely to operate at a reduced capacity until Friday, Mr Hammond said.
In a statement, BAA said: "Passengers should anticipate further delays and cancellations in the following days and potentially beyond Christmas Day.
It added that "significant cancellations" would extend into at least tomorrow, telling passengers to keep away from terminals unless they knew for sure their flight was operating.
The airport's second runway continued to be on shutdown today.
Many areas of the airfield "will not be usable until Wednesday morning at the earliest", the operator said.
At Gatwick 150 staff were working to clear snow and de-ice the planes overnight in the hope of reopening later today.
The big freeze has not only affected those trying to leave the UK by air.
After yesterday's half mile-long queues at St Pancras, Eurostar urged people to only come to the station if they had a confirmed booking for travel today.
"Thank you for your patience," the operator added.
The UK's roads and rail network has also been badly hit.
Giving a statement to the Commons yesterday, Mr Hammond said disruption was "inevitable" given the severity of the weather conditions and that the transport system would "struggle to recover" in the days leading up to Christmas with more poor weather expected.
The strategic road network and rail network have performed "broadly satisfactorily", he said.
But he appeared to take a swipe at the reaction of BAA.
"The experience at airports, and Heathrow in particular, has however been different," the Transport Secretary said.
Speaking later on BBC News, he said airport operators had a "responsibility to make things work better".
Mr Hammond added: "Things did not go well at Heathrow, there's absolutely no getting away from that."
Yesterday, BAA announced that it was unable to accept any more people in the "extremely congested" Terminals 1 and 3.
The airport operator apologised to customers, but defended its reaction to the adverse conditions.
"Our staff are working around the clock to do everything they can to help," it said.
BAA chief executive Colin Matthews has promised a thorough investigation into events at Heathrow.
"When we have got every passenger and every bag where they want to be, we will crawl all over this incident to find out everything that we should learn from it," he told Channel 4 News.
"If we need to spend money on new equipment, we will do so."
Forecasters believe the end of the big freeze is in sight, with the long-awaited thaw expected to begin on Boxing Day.
Paul Mott, forecaster at MeteoGroup, the Press Association's weather division, said today should be a dry day for much of the country, but it was unlikely that the mercury would climb sufficiently to ensure a thaw.
"There will be no melting over the next few days, but we will see milder weather after Christmas. And by December 27, there should be significant thawing," he said.
British Airways said it was suffering significant disruption to its flights in and out of Heathrow.
The airline said: "With the second runway at the airport not being available due to the continuing poor weather we have had to make a significant number of cancellations, especially to our short-haul programme.
"We are extremely sorry for the inconvenience. Our staff are doing everything they can to help you."
Eurostar said it was running a restricted service due to the continued bad weather.
"We are asking all customers booked to travel before Christmas to refund or exchange their tickets free of charge, if their travel is not essential," the cross-channel operator said.
Queues of Eurostar passengers were already building up at St Pancras today.
The coldest temperature in the UK overnight was in Crosby, Merseyside, which sunk to minus 17.6C (0.3F).
Shap in Cumbria recorded minus 15.4C (4F), along with Capel Curig in north Wales.
It was similar temperature at Tyndrum in the Scottish Highlands.
Central London saw a much milder night, with the mercury hovering just above 0C (32F).
Gatwick safely reopened its runway at 6am, and the first departure was a Monarch flight to Innsbruck which took off at 6.06am.
Passengers should expect further disruption today, with delays and cancellations inevitable, Gatwick said.
Passengers planning to travel through Gatwick today, and for the rest of the week, should check the flight status on their airline's website first and only make their way to the airport if their flight has been confirmed.
Flight information is also available on the Gatwick website at www.gatwickairport.com and regular updates will appear on Twitter @Gatwick-Airport.
Six hundred flights are scheduled at Gatwick today, and around 100,000 passengers are expected to travel through the airport.
Blizzards take toll on Ulster's roads
Sub-zero temperatures have left Northern Ireland’s roads in their worst condition in a quarter of a century, according to the Roads Service.
Colin Brown, the Roads Service NI Network manager, said the cold spell produced “the worst prolonged conditions” he had seen in 25 years.
“It's very dangerous — there's now a carpet of snow on top of a frozen surface, so it's like a three-layer scenario,” Mr Brown said.
“Forecasts are indicating the sub-zero temperatures are set to continue until Christmas and we would appeal to drivers over the holiday period to slow down and drive carefully. Staff will be continuing to work over the Christmas period to try and keep the main network open.”
The Met Office expected the minimum temperature last night to fall to -15C. All main roads across Northern Ireland are passable but with care.
On many roads lane widths are restricted and in Armagh snow and slush is common between wheel-tracks even on main roads. Private plough contractors have been deployed in the north and in the Maghera and Glenshane Pass area to provide additional resources to remove snow from roads.
In Fermanagh, the south-east of the county is the worst affected. Main roads are accessible with care but hilly roads are impassable.
The road conditions have also severely disrupted the bus network, particularly Ulsterbus services, many of which have been cancelled.
In Belfast Metro services are operating via main roads only and have experienced severe delays.